How to easily and safely level up your martial arts

1. If you're not yet a slave, choose any slave camp in the desert. When choosing, pay attention to the stats of the strong samurai guarding the nobles; the higher the stats, the better.
   If you're already a slave, forget about it.
2. Beat up everyone in the camp. If your character is new and hasn't yet leveled up your fortress, practice on the regular guards first. Once your fortress is at least level 15, head to the samurai. They'll still heal you, of course, but they can kill you with one blow, especially if you're naked. It's also in your best interest to keep your limbs intact, so as soon as your legs or arms drop below half health, run to your cage.
3. You can also equip shackles on your character and fill their entire inventory with them to increase carrying capacity and strength. Simply break them and discard them while you're in the cage. Caring slave masters will re-equip them for you. You can also steal slavers' backpacks to apply debuffs to your character and fill them with shackles.

When you get bored or lose a limb, steal food from a bar or a merchant, eat your fill, and run away.